Given the total amount of money collected for the number of items sold, we can work out the exact amount collected for each type of item; shirts and dresses.

Knowing that there were also 3 times as many shirts sold, there were 3 units of shirts sold, and 1 unit of dresses sold. If we were to compare 1 unit of shirts, and 1 unit of dress, it would also be the same as comparing the same quantity for each type of item.

1 unit of shirts will be equivalent to $450 collected, since 3 units is $1350 collected.

1 unit of dresses will be equivalent to $630 collected in this case.

To compare 1 unit of shirts and dresses each ( that means same number of shirts and dresses!), we can see that there is $180 more collected from the dresses (630 – 450).

We know that for each dress, $12 more was collected, compared to each shirt. So, if $180 more was collected for each unit of shirts and dresses, if we divide $180 by $12, we know that the answer is 15. That means that there were 15 items per unit.

So, from here, we know that 15 dresses costs $630, 1 dress would be $42.